Diagnosis of vascular diseases using three-dimensional imaging requires visualization of the blood flow through the corresponding vessels. Treatment generally takes place with minimal invasion using catheters, which are inserted into the corresponding blood vessel.
In order to be able to plan a minimally invasive intervention as precisely as possible and in particular to be able to carry it out exactly, the physician requires information about the position and spread of the vessels (location information) as well as the most accurate information possible about the blood flow through the corresponding vessel (temporal information). While aneurysms generally show up very clearly in the corresponding images, stenoses are generally relatively difficult to see. Instead the angiograms show points in the vessels where the through-flow of blood is much reduced. If stenosis leads to the complete occlusion of a vessel, this means that the corresponding vessel and all the vessels supplied by it are no longer identifiable in the x-ray recording. The three-dimensional visualization of the blood flow therefore provides the physician with important information about the degree of constriction or widening of a vessel and any possible effect on other vessels.
In a clinical situation the diagnosis of vascular diseases is currently based on temporal two-dimensional angiography sequences (showing the blood flow) or static three-dimensional data sets, which generally show a completely filled vessel tree.
It is of considerable advantage if the temporally dynamic blood flow is known in the three-dimensional as well as the two-dimensional. To determine the blood flow in the three-dimensional volume, various approaches are known in the prior art.
A first approach consists of simulating the blood flow in the three-dimensional volume. These simulations operate without observing a true flow. Therefore it is a pure simulation. Only the three-dimensional volume data set is required for the simulation. In the context of the simulation, the flow movement through the vessels is calculated based on physical laws. The simulation is based on the Navier-Stokes equations, which allow a numerical approximation of so-called reactive flows. The Navier-Stokes equations form a complex of differential equations, which represents the laws of physics. Essentially they are based on the conservation equations for mass, momentum, energy and in some instances rotational momentum. During the simulation the viscosity and density of the blood are taken into account as are the influences of external pressure on the vessel. This type of simulation is known to those skilled in the art as computational fluid dynamics (abbreviated to CFD). By applying the Navier-Stokes equations to a specific vascular system it is possible to achieve a physically correct simulation of the blood flow, subject to sufficiently precise calculation and adequate information about the vascular system and other ambient conditions.
The overall complex of Navier-Stokes equations in conjunction with flow simulation is described for example in T. Petrila, D. Trif, “Basics of Fluid Mechanics and Introduction to Computational Fluid Dynamics, Numerical Methods and Algorithms”, Springer-Verlag, 2005.
The procedure described to date is based on the real three-dimensional volume data set. However it represents a pure simulation in respect of the temporal dynamics of the blood flow. There is no feeding back to reality.
In another known procedure only two-dimensional images are used. Here two-dimensional angiography sequences are generated from a view with constant C-arm alignment as a contrast agent is briefly injected. The angiography sequences show the temporal propagation of the contrast agent through the required vessels. Generally a reference image without contrast agent is acquired at the start of the sequence and this is subtracted from all subsequent recordings in the sequence, in order to see only the part of the vessel tree filled with contrast agent in the images. The method is also known as digital subtraction angiography (DSA). However the two-dimensional angiography sequences only supply information with local two-dimensional resolution, not information with spatial (=local three-dimensional) resolution.
DE 10 2004 018 499 A1 discloses a determination method of the type mentioned in the introduction. With this method the computer uses the group of x-ray images assigned to the respective acquisition time and the volume data set to determine a respective possible presence distribution for each acquisition time. The computer also uses the temporal sequence of the presence distributions and a vascular structure of the vascular system to determine a final presence distribution for each acquisition time.
In some instance it is possible to use the procedure known from DE 10 2004 018 499 A1 to map the blood flow correctly from the two-dimensional to the three-dimensional. The procedure from DE 10 2004 018 499 A1 therefore already has significant advantages compared with the locally purely two-dimensional processing of the angiography sequence.